What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Donation Amount is available in the Paypal Payment tool.
You can see this option available during the configuration of the Paypal payment wizard.
You can find out more about order types form on following link:
And here is a JotForm template http://www.jotform.com/form-templates/donation-form-2 that have a basic example of the Custom Amount/Donation using Paypal tool.
Hope this will help. Let us know if you need further assistance.